The head of the taskforce, Jan Egeland, said implementing the plan mostly relied on Damascus, which has still not given clearance for UN convoys to reach six of the country's 18 besieged areas.
"I do not know why they will not give permission," Egeland said. "It is a violation of international law to prevent us from going."
Most of the areas concerned are besieged by the Syrian army, not the rebels, Egeland said, adding that the taskforce submitted an operations plan to the government at a meeting on Thursday.
"Altogether, the aim is to reach an impressive 1.1 million people before the end of April," he said.
The UN human rights chief Zeid Ra'ad Al Hussein had warned last month that thousands of Syrians risked starving to death in besieged areas, after shocking images spread of starving residents in Madaya.
President Bashar al-Assad's government has granted permission to reach several besieged areas in recent weeks.
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A convoy including the UN, the International Red Cross (ICRC) and Syrian Red Crescent was distributing aid in four areas -- Madaya, Zabadani, Fua and Kefraya -- on Thursday in an operation that was expected to continue into the night.
Aside from food and basic medical supplies, Egeland said vaccinations were a key part of the UN's humanitarian push for the coming weeks.
"The vaccination rate is now in many areas down to 50 or 60 percent which is a proscription for epidemic disease," he told reporters in Geneva, where fragile peace talks to end five years of civil were continuing.
Egeland noted that a ceasefire that has largely held since being declared on February 27 had allowed UN staff to traverse the country with far more security, but that sporadic fighting had still hampered movement.
Since restrictions on the movements of humanitarian workers were eased in January, aid has reached 150,000 of the nearly half million people living in besieged areas.
That number has meanwhile stagnated since the beginning of the month, as the UN waits for access.
The UN is also trying to improve its technical capacity to make humanitarian air drops over Deir Ezzor, where some 200,000 people are besieged by the Islamic State group, Egeland said.
Aid deliveries have meanwhile also reached nearly 109,000 people in what the UN calls hard-to-reach areas in recent weeks.